When you use the home routine, do you have to hold the respective button on all the time untill homed?
The reason I ask is that in the edit mode, it is purely a software command, but in the rest, it involves hardware input which might suggest the Z & Y are being selected simutaneously, possibly by wiring malfunction/short etc.
You never mentioned the control going in to alarm, so it appears the control thinks this is legitimate operation.
Alot depends on just how the machine tool builder implemented the hardware/input,(if he did not interlock anything).
Failing this, it could be a firmware fault on the servo IC's on the MB, but due to working OK in RUN does not seem to point to this.
Most of the harware fault suggestions can be detected by use of the Diagnostic I/O table.